Trump Republicans’ downscale strength in 2016 was an amplification of a decadeslong trend. The core constituency of the Republican Party has been moving downscale for decades, first in response to cultural issues like abortion. The state of Pennsylvania provides examples. Metro Pittsburgh, with its steel-and-coal economy, never warmed to Ronald Reagan; George H.W. Bush, running to succeed him, won only 40% there in 1988. But by 2004 the younger George Bush raised the Republican percentage there to 48%, and Donald Trump carried it with 50%. The Republican percentage in Pennsylvania beyond its two big metropolitan areas remained static, at 58% in 1988, 57% in 2004 and 59% in 2016.

As Newton’s third law says that there is in nature for every action an equal and opposite reaction, so in American politics, for every demographic group trending toward one party, there is usually another with opposite views trending toward the other. In Pennsylvania, the four affluent suburban counties around Philadelphia voted 61% for Bush 41 in 1988, 46% for Bush 43 in 2004 and 41% for Donald Trump in 2016.

The increasingly downscale Republican and increasingly upscale Democratic constituencies are increasingly reflected in policy. While Mr. Trump orders a payroll-tax suspension, with dollars benefits flowing mostly to modest earners, Nancy Pelosi’s Democrats demand increased deductions for state and local taxes, which would mainly favor those with income of more than $650,000.

This economically modest Republican Party is Mr. Trump’s party—for the moment.